(1) Field of the Invention
The present invention relates to an optical receiving device, and more particularly, to an optical receiving device for receiving an optical signal and detecting an interruption of the optical input.
(2) Description of the Related Art
With the advent of a multimedia age involving the Internet, the optical network communication technologies for backbone communication systems are expected to meet demands for higher quality and wider coverage of services and are rapidly advancing to realize an information-oriented society. Also, in recent years, the capacity and transmission rate of optical communication lines are more and more increasing, and as a result, receiving sections of optical transmission systems are required to offer highly advanced functions. For example, the receiving sections are required to issue an alarm signal quickly in case the optical input is interrupted.
In optical receivers for optical fiber communications, APDs (Avalanche Photo-Diodes) having high light reception sensitivity are widely used as light receiving elements. An APD is an optical semiconductor device for converting light into an electrical signal and is used in a state (reverse-biased state) with voltage applied in a direction (cathode→anode) such that there is a higher resistance to the current flow through the diode.
While in this state, the APD is irradiated with light, whereupon the light is absorbed and electrons are excited inside the APD. When the excited electrons with high energy move, these electrons excite other electrons. This phenomenon repeats itself (the avalanche effect is caused), whereby an electrical signal is obtained.
Also, due to the avalanche effect, the APD produces a current multiplication action, which varies depending on the voltage applied to the APD (i.e., the signal amplification factor of the APD varies with change in the bias voltage applied to the APD). Further, the current multiplication action heavily depends on ambient temperature (the current multiplication action has a temperature characteristic).
Thus, to keep the multiplication factor of the APD constant irrespective of temperature variation, the bias voltage needs to be variably set in accordance with the ambient temperature. Also, in cases where the power supply undergoes variation, the bias voltage needs to be kept at a constant value irrespective of the power supply variation.
On the other hand, when the input of the optical signal is interrupted, the current (photocurrent) of the electrical signal converted by the APD approaches zero, with the result that the bias current of the APD also varies. Accordingly, in a conventional procedure for detecting an interruption of the optical input, the value of a control current, which is inversely proportional to the bias current of the APD, is compared with a threshold, and if it is judged as a result of the comparison that the optical input is interrupted, an alarm is issued. Thus, the control current inversely proportional to the bias current of the APD has conventionally been used as information based on which an interruption of the optical input is detected.
In the conventional optical input interruption detection, the control current is used to determine whether or not the optical input is interrupted, but since the bias voltage varies due to variations in temperature and power supply, as mentioned above, the value of the control current also varies. It is therefore necessary that the threshold level used for the detection should also be corrected in accordance with variations in temperature and power supply, and a correction circuit has hitherto been used for the purpose.
Such correction circuits are, however, constituted by analog devices and thus have errors differing from one circuit board to another. Also, in conventional optical receivers, since the correction is performed with respect to a weak control current, different circuit boards on which the respective optical receivers are mounted show different detection levels for detecting an interruption of the optical input, which leads to lowering of the quality and reliability of the optical receivers.
Meanwhile, in recent years, there has been a demand for a shorter response time required from an interruption of the optical input to the issue of an alarm (e.g., within 100 μs according to the Bellcore standard (currently, Telcordia standard)). However, according to the conventional optical input interruption detection, the time required until the issue of an alarm is long, and therefore, the alarm issue time is made to meet the standard by performing a logical operation using a fast alarm issued from a clock data recovery circuit, for example. This leads to an increase in the number of circuit elements and wiring patterns necessary for the logical operation, and thus the conventional optical receivers are not efficiently designed.